Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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38 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM
Continued f rom page 37
COATINGS & LININGS
ESSENTIALS
Damage-Tolerant Sacrificial
Coatings Provide Offshore
Protection
ermal-sprayed aluminum (TSA) sacrificial
coatings have been used in offshore oil and gas
platforms for decades. e low corrosion rate
of aluminum in seawater, coupled with its abil-
ity to act as an anode with respect to steel,
makes TSA ideal for offshore applications. It
provides a barrier layer when intact and ca-
thodic protection (CP) when damaged. e
damage tolerance of TSA coatings is known
from experience, but the level of damage these
coatings can endure and still provide sacrifi-
cial protection is unexplored. Understanding
the effect of large defects or damage on TSA's
performance is important. While TSA would
polarize steel even if damaged, the polariza-
tion may be insufficient to protect the steel or
the dissolution rate of TSA may be high.
To study the damage tolerance of TSA,
NACE International member Shiladitya Paul
with TWI (Cambridge, United Kingdom) and
the University of Leicester (Leicester, United
Kingdom) exposed coated carbon steel (CS)
bar with a high degree of damage to synthetic
seawater and monitored the potential. Paul
observed that TSA polarized the CS bar to
below –800 mV vs. a silver/silver chloride
(Ag/AgCl) reference electrode, even when
damage exposed 90% of the steel surface.
e potential, however, became less nega-
tive with time and reached values close to the
corrosion potential of steel in 35 days. Even
after values close to the steel corrosion poten-
tial were reached, rust was not seen on the
exposed CS surface due to the deposition of a
fine layer of calcareous matter, which implies
that TSA can provide corrosion protection to
offshore CS structures even when extreme
damage occurs. Paul notes, however, that rust
spots began to emerge after 100 days, leading
to the conclusion that extreme damage may
lead to accelerated corrosion of TSA with sub-
sequent reduction of its service life. Further, in
cases where TSA coating with extreme dam-
age is exposed to splash and tidal zones with
only intermittent contact with seawater, the
efficacy of TSA may be limited as continuous
electrolytic contact is essential for CP.
Details of this study can be found in
CORROSION 2018 paper no. 10949, "Cathodic
Protection of Offshore Structures by Extreme
Damage Tolerant Sacrificial Coatings," by
S. Paul.